Just like hair anywhere else, just trimming or shaving cannot make your hair grow back thicker, coarser or whatever other old wives tales are out there. Your hair grows from the follicle inside your skin only. So, trimming your eyebrows on the outside is going to have ZERO effect on your hair follicles. What it DOES do, is due to the fact you are cutting your brows to a blunt end, it may make them LOOK a tiny bit fuller, but you are not going to really make them thicker or fuller just by cutting.